After the brutal hills of the past two days, my third morning features a long, gentle descent. The Bolinas Ridge Trail makes an 11-mile straight shot along a rounded ridge bisecting rural north Marin. And my partners deliver a welcome jolt of energy. Kelly Dunleavy, a petite 23-year-old blogger and runner who competes in regional triathlons and trail races, will go seven miles out, and then double back to her car. Matching her stride is sports nutritionist Sunny Blende, 58, who does ultramarathons and ran 45 miles from one rim of the Grand Canyon to the other and back one month earlier. She plans to run 10 miles with me before turning around. They both say they're looking for an easy pace today, which I assure them fits into my travel plans.

Kelly and Sunny keep up a lively conversation—talking running, races, nutrition, training, ultras—that distracts me from the twitching muscles in my quads. We pass through a few miles of cool, quiet woods, then emerge onto grassy meadows above the bucolic Olema Valley where a forested ridge obstructs our view of the Pacific. The valley appears peaceful enough, but looks deceive: It straddles the San Andreas Fault, where the Earth's North American and Pacific plates grind against one another, gradually sawing off the ice axe-shaped horn of land called the Point Reyes peninsula. So someday this trail we're on could have an ocean view.

About 90 minutes into our run, after Kelly has turned back, the horizon ahead retreats to reveal Tomales Bay, a blue finger of sea giving California a 12-mile-long prostate exam. Rearing up above the bay is tomorrow's objective: the green wall of Inverness Ridge—big, steep hills that, I'm thinking, exist to swallow overconfident and overfatigued runners whole. It looks daunting, and three days of hard running has daunted my ability to conjugate properly.

Surrounded by cows in a field, Sunny and I shake hands. Then I slowly jog the last mile down to Olema, a handful of lodges and restaurants at a sleepy T intersection where 19th-century loggers supported a bustling economy based on saloons and a certain age-old profession. Seeking a more restorative R&R, I sink deep into the hot tub in my spacious room at the Point Reyes Seashore Lodge, nearly falling asleep. In the evening, I hobble across the street to the Olema Inn to meet my wife's 28- year-old niece and her boyfriend, who drove over from Oakland for a couple of beers and a dinner of locally caught mussels, scallops, ahi, and oysters prepared eight ways (with champagne, horseradish and cracked pepper, spicy tomato water and seeds, and four other ways that slip from my memory as quickly as I digest them). It helps me forget—for a little while—the sloshing reservoirs of lactic acid in my quads.

Later, with the cool night air slipping through my room's open French doors, the cumulative effect of the run, hot tub, and feast hits me like I'm one of those actors in an Ambien commercial. But just before nose-diving into a rejuvenating coma of running dreams, my thoughts wander toward the precipice of anxiety over tomorrow's itinerary: running 12 miles across Point Reyes National Seashore, over two mountains, with 2,100 feet of up and down. And right now every synapse in my brain is calling out, "Control tower to Mike: You're cleared for a rest day."

Sheesh, what was I thinking? Tomorrow, the hardest day of the hardest adventure of my running career may bring on my hardest-ever bonk.

Day 4: The Final Climb

Just going up the lodge's half-dozen front-porch steps in the morning, my legs feel like dead tree stumps. It doesn't bode well for today, a harsh reality that crystallizes as my partners set the week's strongest pace—which shouldn't surprise me. Tamalpa President Ken Grebenstein, 55, is a gregarious sort who has run 80 to 100 marathons and ultras, including the 100-mile Western States Endurance Run. With him is Dave Ripp, 56, a three-hour-marathoner with a quieter demeanor and a salt-and-pepper beard who's been running Marin's trails for more than 30 years. Twenty minutes into the steep climb up Mt. Wittenberg, they're already dragging me like a broken tailpipe on an old pickup.

As I'm struggling for breaths, Ken churns out stories about running Marin—of its embracing community, of a trail system so elaborate you can explore for years and still not know all of it, and of 50-mile training workouts.

"Many of my running buddies, like me, moved here for the trails and the friends that you make," he says. "Trail running in Marin is like few places in the world. In the space of a few miles, you can go from redwood forests to sweeping vistas. People fall in love with this place. Their lives get wrapped around it."

Today's trails are breathtaking enough to recharge me, despite how knackered I feel. Digging deep for some newfound reserve of stamina, I chase Ken and Dave over the roller-coaster contours of Inverness Ridge beneath pine and fir trees and a coastal fog blowing in on a cool wind. I can't quite keep up my end of the conversation, but I'm chugging hard on their heels, locating that place where the bonk keeps a merciful distance and I fall into a rhythm of movement and awareness of my body and surroundings that feels sustainable—for a while, anyway.

The relentless climb up Mt. Vision just about sucks the last volts from my battery. I struggle to stay with these guys—indeed, just to keep lifting my feet off the ground. But at the top, the fog abruptly dissolves, casting us into warm, resuscitating sunshine. Pausing to soak up the views—and for me to suck in some oxygen—we look down 1,200 feet to Tomales Bay shimmering in the sunshine, and across it to a chaotic scrum of brown hills, scattered buildings, and narrow roads, a vision of a past, unspoiled California.

I can see something else in this view, something that has me feeling good about where I'm standing, literally and figuratively: It's all down from here. It won't matter when my quads begin to harden like poured cement on the steep, outsole-slapping, thousand-foot drop into Inverness. I'll be able to ignore the cacophony of complaints rising from my hip flexors, lower back, Achilles, calves, and soles, because I've answered the open-ended question hanging over this adventure from its outset: Yes, I'm going to make it. I have succeeded in refining my sense of my own endurance while celebrating the pleasures of great scenery, people, food, and, of course, hot tubs. I've discovered a new way to feed my running jones and see a big chunk of wilderness at the same time by running across it for days.

Most of all, I've taken Janet Bowman's advice: I've embraced the hills. One could run for a lifetime in Marin and never get tired of it.

Only very, very tired.

Check out the Runner's World trail-running section for destinations, gear, tips, blogs, and all other things trail-related, at runnersworld.com/trailrunning.

Trip PlannerResources for doing this run yourself.

This four-day trek goes from Sausalito to Inverness, California, 42 miles, mostly on well-maintained trails, with short stretches on pavement. It passes through Golden Gate National Recreation Area (nps.gov/goga); Mt. Tamalpais State Park (mttam.net); Muir Woods National Monument (nps.gov/muwo); and Point Reyes National Seashore (nps.gov/pore). The trip was created for the author by Wine Country Trekking (winecountrytrekking.com).

Food and LodgingLodging runs from $135 to $499 per night. Dinners are $30 to $60 per person.

TimingThe best months for mild, frequently sunny days and no fog are October (though the hills are brown) and May (when the hills green up and flowers bloom). It's often rainy between December and March. In the summer, mornings offer ideal running temps.

Escape Without IncidentSimple tips to prepare yourself for the trails ahead.

Trail TechniquesLooking two steps ahead helps you anticipate changes in the terrain.A shorter stride will keep your weight directly over your feet as you land, which lessens the chance of a fall.The direct route isn't always the fastest or safest. This is especially true when it runs across wet rocks or slick roots.Walk if the footing gets treacherous or when the trail gets too steep. Moving too fast invites injury.

Smart NavigationChoose trails that are well maintained and clearly marked, as many are in national and state parks.Pay attention—don't just follow a partner blindly.Carry a map and consult it at key turns and landmarks to keep track of your location.At junctions, hold your map to match the relative position of the trails ("Trail A is to my left, Trail B straight ahead," etc). This will help you know which path to take.

Risk AssessmentKnow beforehand whether you'll get cell-phone coverage or see other people (to get directions or summon help).Carry energy bars or a sports drink on longer runs, as well as an extra layer of clothing and a compact headlamp—just in case.Make sure your car keys are always in a secure pocket where they won't fall out.Apply sunblock and wear a hat and sunglasses if you're climbing to higher elevations.